The invention described herein arose in the course of, or under, Contract No.W-7405-ENG48 between the United States Department of Energy and the University of California for the operation of the Lawrence Livermore National Laboratory.
This invention relates to magnetically retained electrical shielding. More particularly, this invention relates to the provision of magnetically retained shielding for an opening to an electrically shielded enclosure containing electronic components, to prevent the passage of electric fields into or out of the enclosure through the opening.
Many electronic structures are housed in electrically shielded cases or enclosures which are constructed of electrically conductive material, such as metal, to provide electrical shielding either to prevent escape of electromagnetic radiation generated by the electronics within the enclosure or to prevent the incursion of stray electromagnetic fields from external sources. Typical of such electronic structures, for example, are computers and the electronics for the generation of laser beams.
A problem, however, arises when rapid and frequent ingress to the interior of the shielded enclosure is required. While a removable cover can be provided, attempts to make such cover easily removable, yet at the same time, provide the requisite shielding, particularly at the joint between the enclosure and the cover, have met with less than satisfactory results.
If, for example, a shielded cover, i.e., a cover constructed of electrically conductive material, is secured to the enclosure by a plurality of fasteners such as bolts, to thereby provide a fit between the cover and enclosure through which undesired leakage of electromagnetic radiation cannot pass, removal of the cover will be slow and cumbersome, which can result in the temptation by operators to omit the use of some of the fasteners. This, in turn, will defeat the leakproof design of the electrical seal, thereby allowing some electromagnetic radiation to pass through the joint between the cover and enclosure either into or out of the enclosure. Even when such a clamping type structure is modified by proving electrically conductive elastomeric materials between the cover and the enclosure, to reduce the number of required fasteners by providing some degree of relief for non-perfectly matching surfaces between the cover and enclosure, it has been found that when such elastomeric materials contained sufficient conductive fillers to provide the requisite electrical sealing or shielding, the durability of the material is reduced to an unsatisfactory level.
The provision of latches to provide rapid removal of the cover and ingress to the interior of the enclosure has also been proposed. However, in some instances, the provision of latches adds unacceptable bulk to the structure, particularly when space is at a premium, for example, when high voltage components are used requiring large air gaps for high voltage standoff in the enclosure.
The use of metallized Velcro on the mating surfaces of the enclosure and removable cover has also been proposed and tried. However, the portions of the Velcro secured to horizontal surfaces are susceptible to collecting dust, lint and other particulate matter. Furthermore, air gaps in the Velcro permit electrical leaks, i.e., result in leakage of electromagnetic radiation through the joint. Furthermore, when high voltage is used in the enclosure, e.g., in electronics used for the generation of a laser beam, such leakage can, in turn, cause arcing and vaporizing of the metallized portions of the Velcro, leading to further degradation of the quality of the electrical seal between the enclosure and the cover.
Metal to metal friction type sealing between the cover and enclosure has also been proposed and tried wherein a tapered surface on either the cover or the enclosure was squeezed into a channel like structure on the other member. However, the frictional forces required to make such a seal sufficient to prevent the undesired electrical leakage, in turn makes subsequent removal of the cover difficult due to the high frictional forces induced by design into such a structure.
It would, therefore, be desirable to provide a structure comprising an electrically shielded enclosure containing electrical components, and a shielded cover, wherein the cover could be secured to the enclosure in a manner which would permit ready access to the interior of the enclosure, yet provide an electrical seal therebetween capable of preventing the leakage of electromagnetic radiation either into or out of the enclosure when the cover is secured to the enclosure.